The present invention relates to nuclear power plant systems and more particularly to a nozzle penetration arrangement for a nuclear reactor pressure vessel closure head, such as a control rod drive mechanism (CRDM) guide tube nozzle penetration, and methods of making them.
A pressurized water nuclear reactor (PWR) includes a lower reactor vessel with a reactor core and an upper control rod assembly, part of which can be lowered into the reactor vessel for controlling the reaction rate of the nuclear reactor. The control rod assembly contains a plurality of vertical nozzles which penetrate the upper cover of the vessel, or closure head, and houses extensions of a control rod, that can be lifted or lowered by a control rod drive mechanism (‘CRDM’), which generally operates by some combination of electrical, electromechanical, hydraulic, or pneumatic motors or drivers. For further details of the design and operation of pressurized water reactors the reader is referred to Chapters 47 and 50 of Steam/its generation and use, 40th Edition, Stultz and Kitto, Eds., Copyright ©1992, The Babcock & Wilcox Company, the text of which is hereby incorporated by reference as though fully set forth herein.
As shown in FIG. 1, designs for a PWR closure head assembly 5 presently used throughout the industry include a reactor head flange 10 which surrounds and may be integral with closure head 20 that forms a hemispherical, dome-shaped pressure boundary. Control Rod Drive Mechanism (CRDM) guide tubes 30, also referred to as the CRDM housing, CRDM nozzles, mech housing or Control Element Drive Mechanism (CEDM) nozzles, pass through and are attached to closure head 20. A stainless steel flange or adaptor 40 is situated at the upper end of the guide tube 30 attachment of each CRDM or CEDM, via full penetration weld 70 shown in FIG. 2. A PWR closure head assembly 5 is a large, heavy structure, typically about 17 feet in diameter and weighing about 90 tons, and includes between 30-100 CRDM guide tubes 30.
Referring to FIG. 2, guide tube 30 is manufactured separately from closure head 20, and then installed in bore hole 22 extending through closure head 20 from concave inner surface 24 to convex outer surface 26. As shown in FIG. 2, guide tube 30 protrudes beyond inner surface 24 and outer surface 26. Closure head 20 is typically fabricated from low-alloy steel and provided with a corrosion resistant cladding 80, such as 308/309 stainless steel, at inner surface 24.
Guide tube 30 is attached to closure head 20 by welding the guide tube 30 to closure head 20 with a partial penetration weld 50 referred to as a ‘J’ groove weld. Guide tube 30 is typically fabricated from Inconel Alloy 600 or Inconel Alloy 690, in which case weld 50 is made using Inconel weld consumables. Partial penetration J groove weld 50 is made between guide tube 30 and a J groove weld preparation profile 52 formed at inner surface 24 and typically covered with a previously heat treated Inconel overlay, in what is known as J groove buttering 60. The previously heat treated J groove buttering 60 allows welding of the guide tube 30 to the buttering 60 without subsequent heat treatment of the J groove attachment weld 50.
J groove attachment weld 50 and the associated guide tube 30 have experienced life limiting degradation in the vicinity of the J groove attachment region attributed to stress corrosion cracking (SCC). This has forced the repair, replacement or inspection of the Inconel J groove weld 50 and guide tubes 30. This degradation has become a commercial and safety concern for all operating PWR stations. A reactor closure head assembly which eliminates the J groove attachment welds between the guide tubes and the inner surface of the reactor closure head would therefore be welcomed by industry.